1. Field of the Invention
The invention relates to a floating-gate memory cell, i.e., a memory cell comprising a gate electrode which is floating with respect to potential and which, by way of injecting hot charge carriers through a thin oxide layer between the substrate crystal and the electrode, is charged or discharged, with the threshold value of a memory transistor designed as an insulated-gate field-effect transistor, being shifted. The writing, for example is effected by way of injecting hot electrons, and the erasing is effected by injecting hot holes.
2. Description of the Prior Art
Memory cells of this kind are known from the following articles:
(a) W. M. Gosney, "IEEE Trans. on Electron Devices", May 1977, pp. 594 to 599, "DIFMOS-A Floating Gate Electrically Erasable Nonvolatile Semiconductor Memory Technology" PA1 (b) T. Ito et al, "IEEE Trans. on Electron Devices", June 1979, pp. 906 to 913, "Low Voltage Alterable EAROM Cells with Nitride-Barrier Avalanche-Injection MIS (NAMIS)".
The memory cell known by the acronym DIFMOS for dual injector floating-gate MOS, has on one hand the following drawbacks:
For its manufacture, there is employed the AL-gate p-channel technology, so that for one cell there is required a relatively large surface area (25,800 .mu.m.sup.2).
It requires separate diode structures with a special doping, i.e. a P.sup.++ /N.sup.+ structure for the electron injection, and a N.sup.++ /P.sup.+ structure for the hole injections.
It is not compatible with the modern n-channel Si-gate technology.
On the other hand, the "NAMIS" memory cell is known to have the following drawbacks:
The injection takes place within the channel area of the memory transistor, so that in a relatively short period of time, the characteristic of the memory transistor becomes changed owing to charges trapped in the gate insulator.
Between the floating-gate electrode and the channel area there is used an extremely thin thermal nitride (9.5 nm) which, for its manufacture, requires a thermal nitration process requiring high temperatures ranging between 1,200.degree. and 1,300.degree. C. which are not compatible with modern standard technologies.